1. Field of the Invention
The invention relates to apparatus and method for use in the manufacture of flextensional sonar transducers and in particular to a method for extending the major axis of flexural shells therefor to allow insertion of piezoelectric elements.
2. Discussion of Prior Art
Commonly, flextensional sonar transducers comprise one more stacks of ceramic piezo-electric elements held in compression along the major axis of an elliptical flexural shell made of filament-wound glass, carbon fibres or metal. In operation, electrical signals applied to the piezo-electric elements produce contractions and extensions along the major axis of the flexural shell. These in turn produce larger transverse flexing movements along the minor axis. In use, hydrostatic pressure on the external wall of the elliptical flexural shell reduces the pressure on the transducer stacks along the major axis. Thus, in order to prevent movement the stacks are assembled in the elliptical shell such that a pre-stress is applied along the length of the stacks. This pre-stress is designed to overcome hydrostatic pressure effects corresponding to the maximum pressure depth of the flextensional transducer.
Conventionally during manufacture of flextensional transducers the elliptical shell is made by first winding a resin-coated filament around a mandrel and then the piezo-electric drive elements are inserted along the major axis of the elliptical shell by applying a compressive force on the minor axis of the shell by means of two flat parallel plates to cause an extension along the major axis. The ceramic drive (piezo-electric elements) together with pre-tensioning wedges are then inserted and finally the compressive force on the minor axis is removed.
A major drawback associated with this method has been the occurrence of interlaminar shearing in the composite shell during the compressive stage of ceramic drive assembly. Conventional shell loading has proved particularly unreliable for deep water transducers, requiring large shell compression.